1. Field of the Invention
The invention relates to improvements to optical aiming systems used in conjunction with excimer laser eye surgery systems, and more specifically, to a method and apparatus for improving the effectiveness of such an optical aiming system in locating points on the eye.
2. Description of the Related Art
The advent of the excimer laser opened a whole new realm of possibilities for eye surgery, providing a non-invasive technique for resculpting the surface of the eye itself to match a desired curvature. Such systems are well known in the art, and are further described, for example, in the inventor's PCT Application PCT/EP93/02667 which is hereby incorporated by reference, as well as in various patents to L'Esperance, such as U.S. Pat. No. 4,665,913.
To improve the accuracy of these excimer laser surgical devices, it is preferable to precisely place each shot from the excimer laser at the desired location. A number of techniques and devices have been developed to achieve this end. For example, a simple fixation light has often been used. Patients fixate their gaze upon that light, generally lessening slow eye movement. This technique does not, however, prevent rapid movements of the eye. Further, a momentary lapse in fixation could result in an ablation shot far from the intended shot location. As an alternative, physical fixation devices have been used which immobilize the eye by physically connecting to the eye, thereby holding it steady.
A more recent technique involves the use of computer aided eye tracking devices. These are optical or topographic location systems that typically use a video camera to either optically or topographically locate and track the center of the eye. Each shot can then be placed at any desired location on the eye relative to that center. Examples of such systems can be found in U.S. Pat. Nos. 5,098,426 to Sklar et al., 5,162,641 to Fountain, and 4,848,340 to Bille. These systems use various techniques to track the center of the eye, such as a computer mapped digital image from a video camera. For example, U.S. Pat. No. 5,098,426, to Sklar, et al., hereby incorporated by reference, describes an eye tracking system that generates a three dimensional profile of the eye and tracks movement by noting changes in that profile. The Sklar patent shows an eye tracker using a slow control loop and a fast control loop. The slow control loop relies on a video camera to provide topographical information that the eye tracker then uses to aim the system optics.
An alternative eye tracking system is shown in U.S. Pat. No. 4,848,340 to Bille, also incorporated by reference. The Bille patent shows a strictly optical, rather than topographical, based system that tracks a reference grid which has been ablated into the eye.
Another eye tracking system using infrared light to illuminate the pupil of the eye has been announced by ISCAN, Inc. This system is described as using infrared light to illuminate the eye, with the system then returning positioning information to a variety of applications, such as computer control through eye movement and assistance to the disabled.
Any of a various number of techniques for locating objects can be readily adapted to locate the center of the eye. It would be desirable, however, to improve the effectiveness and accuracy of such systems. That is, given an object location system used in conjunction with an excimer laser system, it would be desirable to provide other improvements that enhance the ability of those systems to accurately locate the center of the eye and provide for accurate aiming of the pulsed excimer beam onto the eye.
These eye tracking systems are not without problems, however. First, misalignment of the optics can result in offsets of where each excimer laser shot actually falls relative to where it should fall. For example, servomotors can be slightly miscalibrated, resulting in these offsets. It would thus be desirable to provide a method and apparatus for eliminating the effects of such miscalibrations.
Second, these systems tend to be either invasive or complicated, in the sense that they require actual physical markings to be made on the eye, as shown in the Bille patent, or require highly complex topographical location systems and multiple feedback loops for locating the center of the eye, as shown in Sklar patent. Thus, simpler methods of providing a reference to the center of the eye would be desirable.